In order to automatically align a high-frequency reflector antennae in the direction of its signal source in the fields of radio technology, communication technology and defence technology, it is known to guide these to the target, which is movable relative to the antenna, or to the signal source with the aid of mechanical or opto-electronic gyrocompasses. Compass-based tracking however has the disadvantage that the location of the target or the signal source must either be known or at least must be predictable in order to be able, from the compass information, to aim at the location of the target or the signal source. Apart from compass-based target tracking or signal source tracking, it is further known to cyclically or in repeating patterns vary the directional characteristic in the antenna diagram of high-frequency reflector antennae and, from the correlation of the received signal behaviour, derive directional information on the target, which spatially changes relative to the antenna alignment, or on the signal source, which locally changes relative to the antenna alignment.
The German laid-open specification DE 198 48 202 A1 has disclosed a high-frequency reflector antenna, which comprises, in the immediate vicinity of a sub-reflector, a mechanically circulating passive element, which deliberately interferes with the direction-dependent reception characteristic in the antenna diagram of the entire antenna system. Insofar as an aimed-at signal source or an aimed-at target lies in the centre or in the focus of the antenna array, interference from the circulating element does not cause a noticeable change in the received signal because the intensity distribution of the received signal, which is in focus, comprises circular-symmetrical properties. But if the target or the signal source is arranged outside the focus of the reflector antenna, which means that the antenna array is wrongly aligned, the received signal strength of the antenna array correlates with the momentary position of the circulating interference element. In the short moment in which the interference element, viewed from the central horn, covers the direction of the target or signal source, the received signal strength reduces, and when the circulating interference element lies outside the direction of the signal source, the received signal strength increases again. With a circulating interference element, the strength of reception is thus cyclically varied, and a mechanical modulation of the received signal is taking place. Using the mechanically circulating passive interference element leads to useful results which may be used for automatic target or signal source tracking. Nevertheless, the constant presence of the circulating interference element means that there is constant signal reception interference which cannot be switched off, thereby unnecessarily reducing reception quality. With strong signal sources, the deliberately introduced interference can be tolerated. However, with weaker signals or signals which can easily be interfered with, this kind of generating a tracking signal is less suitable.
Since the circulating interference element according to the DE 198 48 202 A1 mentioned in the beginning is arranged in the immediate vicinity of the sub-reflector, the geometric dimensions and thus, the interfering properties of the interference element must be selected very carefully, because in the near and medium field area of the horn, the electrical and magnetic vectors of the received signal are no longer perpendicular to each other, and it is very complicated to theoretically model the electro-magnetic wave properties in this range of the high-frequency reflector antenna, making any predictions very difficult. In the direct vicinity of a horn of generally a high-frequency reflector antenna therefore, the interference effect of an interference element is difficult to predict, and a very small change in the properties of the interference element may cause very big changes in the interference effect.
According to the teaching of the German laid-open specification DE 100 41 996 A1, the method according to the DE 198 48 202 A1 mentioned in the beginning was further developed. Instead of a mechanically circulating interference element which is constantly situated in the near-field area between horn and sub-reflector, a stationary arrangement of elements, specifically selected for a certain pre-selected polarization of the received signal, was proposed, wherein these elements are electronically switchable. To this end, according to the teaching of the DE 198 48 202 A1, an array of small electronically switchable dipole antennae are positioned in the beam path in the medium field area between the main reflector and the sub-reflector, i.e. in the beam path at a certain distance from the horn. The small dipole antennae may, for example, be switched on and off via a PIN diode in resonance condition with the received signal. Because the electronically switchable dipole antennae are activated in turn (one after the other) the antenna diagram of the high-frequency reflector antenna is deliberately changed. This change in the directional characteristic, which is circulatory and electronically switchable, can then be correlated with an internal synchronized electronically circulating vector signal together with the variation in received signal strength. From the correlation of the interference elements, which are locally activated over time, with the synchronously varying target signal strength or received signal strength, directional information as with the mechanically circulating interference element may be derived, in which there is a target or a signal source which is outside the focus of the high-frequency reflector antenna. This further developed high-frequency reflector antenna has the advantage that the interference elements, the electronically switchable dipole antennae, can be activated and deactivated electronically. Nevertheless, use of this high-frequency reflector antenna is limited to a once preselected polarization of a transmit signal. For the reception of a differently polarized signal source, it is therefore necessary to mechanically alter the electronically switchable elements between the sub-reflector and the main reflector and to align them with the new polarization.
A high-frequency reflector antenna which is used for simultaneously receiving and transmitting, comprises differences in the near and medium field area in the spatial output densities of the high-frequency field between reception and transmission, which differ by up to 120 dB. Insofar as only the reception for directional detection shall be influenced, the arrangement according to the teaching of the DE 100 41 996 A1 suffices. But if the high-frequency reflector antenna is simultaneously or alternately switched into transmission mode, the electronically switchable interference elements and/or the directly adjacent electronic connections may also receive the transmission output of the high-frequency reflector antenna in an undesirable manner. It is therefore necessary to be extremely precise in selecting the spatial positioning of the electronically switchable interference elements. As early as minor changes occur in the spatial position of the electronically switchable interference elements, for example when vibrations occur or if improper adjustments are made to the high-frequency reflector antenna, the wrongly positioned, electronically switchable interference elements, due to the high transmission output, may receive, at best, the transmission output in an undesirable manner and feed it back into the antenna electronics and, at worst, destroy the electronics of the high-frequency reflector antenna.
Experimental measurements taken of the field properties in the medium-field range between the main reflector and the sub-reflector of a high-frequency reflector antenna have resulted in the fact that the arrangement of interference elements in this spatial area leads to usable results for a stable mechanical arrangement and comparatively low sensitivity of the electronically switchable interference elements towards an increased transmission output for a small unintentional misalignment. The medium-field range between the main reflector and the sub-reflector, in relation to the placing of electronically switchable interference elements, is however not suitable for the accommodation of electronically switchable interference elements which are equally suitable for various polarizations of the signal source.
In the German laid-open specification DE 10 2007 007 707 A1 the use of immovably arranged, controllable radiator elements for influencing the directional characteristic of reflector antennae is disclosed. The radiator elements are arranged in the medium-field area of the horn in the beam path between the sub-reflector and the main reflector. The possibilities for influencing the reception characteristic of the reflector antenna in a direction-dependent manner in the near-field area which is very sensitive, even to small interferences, are very limited.
According to the teaching of the U.S. Pat. No. 4,387,378 A the direction-dependent reception characteristic for an antenna with main reflector and horn can be influenced by arranging rod-like elements with adjustable reactance in the horn. However, it is not possible to use these elements for exerting a polarization-specific influence.